The present invention relates generally to pozzolanic cements, and more specifically relates to a white pozzolan derived from glass manufacturing by-products, and to the method for producing the white pozzolan. The invention relates as well to cement compositions based on the white pozzolan, such as white and pigmented blended pozzolanic cements of high durability for use in applications such as white or colored architectural concrete, building materials, and manufactured cementitious products. Although the white color of the pozzolan and its consequent use with white cement can generate great added value, the white pozzolan also functions as a high performance pozzolan with grey cement.
As used herein the following definitions shall apply, which are adopted from ASTM C-618: Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete:
xe2x80x9cPozzolanxe2x80x94A siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
Class N Pozzolanxe2x80x94Raw or calcined natural pozzolans that comply with the applicable requirements for the class as given herein, such as some diatomaceous earths; opaline cherts and shales; tuffs and volcanic ashes or pumicites, calcined or uncalcined; and various materials requiring calcination to induce satisfactory properties, such as some clays and shales.
Class F Fly Ashxe2x80x94Fly ash normally produced from burning anthracite or bituminous coal that meets the applicable requirements for this class as given herein. This class fly ash has pozzolanic properties.
Class C Fly Ashxe2x80x94Fly ash normally produced from lignite or subbituminous coal that meets the applicable requirements for this class as given herein. This class fly ash, in addition to having pozzolanic properties, also has some cementitious properties.xe2x80x9d
In a representative glass fiber manufacturing facility, typically 10-20 wt % of the processed glass material is not converted to final product and is rejected as industrial by-product or waste and sent for disposal to a landfill. This rejected material represents a substantial cost to the industry and also generates a consequent detrimental impact on the environment. While the rejected by-product referred to may have widely varying physical form, ranging from thick fiber bundles to partially fused fiber agglomerates and shot, it is evident from chemical analyses of various samples recovered at different times, that the material still has a substantially constant chemical and mineralogical make-up. Thus, unlike wastes from many other industrial processes which typically have widely varying chemical and mineralogical properties, the waste from the glass fiber manufacturing process is very consistent in composition and still benefits from the stringent engineering quality control applied to the glass-making process itself. This consistency is a major advantage to any potential utilization of the glass fiber manufacturing waste.
More specifically, the glass formulations of great relevance to this invention are those of low alkali calcia-alumina-silica compositions (CaOxe2x80x94Al2O3xe2x80x94SiO2 or xe2x80x9cCASxe2x80x9d) typically used for commercial glass fiber manufactured to comply with ASTM D-578. These formulations are given in Table 1. The compositions are vitreous and by virtue of their components have very low levels of discolorants. These compositions are expressed conventionally in terms of the element oxide and are not meant to imply that the oxides, crystalline or otherwise, are present as distinct compounds in the amorphous glasses.
Several features are immediately evident from inspection of the data in Table 1. First, the general chemical and mineralogical composition of the glass fiber material is very similar to amorphous (glassy) calcium alumino-silicate materials, such as blast-furnace slag and Class C fly ash, that are commonly used as cementitious or pozzolanic admixtures in portland cement concrete; second, the alkali (Na2O+K2O) content of the glass is very low (0 to 2%); and third, with their inherently low iron contents (0.05 to 0.8%), the glasses have little or no color. Low alkali content and chemical consistency differentiates the glass fiber manufacturing waste from post consumer waste glass, for example container bottles and flat glass, that have widely varying chemical composition, generally high alkali content, and in the case of container/bottle glass are highly colored.
Conventionally, white portland cement is used in a variety of applications, including but not limited to: white or light colored architectural concrete; precast concrete panels; cast stone monuments and statuary; ornamental landscaping; decorative flooring tiles and terrazzo; wall cladding, stuccos and plasters; tile grout; caulk and white cement paint.
White portland cement by itself does not have good durability, particularly under service conditions where it is exposed to attack by sulfate solutions and other aggressive chemicals. This is because the chemical composition of white cement is different from gray Portland cement in order to obtain the desirable white color. The main difference is that white cement has a very low iron content which during the manufacturing process leads to the formation of much higher tricalcium aluminate C3A content in the finish clinker. Typically during cement manufacturing, C3A reacts with iron oxide to form tetracalcium aluminoferite (C4AF). The lack of iron oxide in white cement results in high levels of tricalcium aluminate that are the reason for the well known susceptibility of white cement to chemical deterioration when exposed to an environment that is rich in sulfate. Such an environment can be found in many soils and in seawater. A high C3A content can also contribute to the increase in volume changes that can result in the formation of cracks in hardened concrete.
Cementitious and pozzolanic admixtures used with portland cement, such as blast-furnace slag, fly ash, silica fume and metakaolin, are characteristically fine particulate powder materials that are comparable in fineness to portland cement. In addition to improving the economics of production through cement replacement, these xe2x80x9csupplementary cementing materialsxe2x80x9d are also well known to improve the long term durability of cement and concrete products, for example by reducing deterioration due to attack by aggressive chemical media, such as sulfate, and expansion due to reactions between the aggregates and the cement alkalis (so-called xe2x80x9calkali-aggregate reactionxe2x80x9d or AAR).
These pozzolans, however, have chemical components that inevitably impart an undesirable dark color to white cement that negates the reason for using the material. For this reason, use of the architecturally desirable white cement has been somewhat limited to applications where there is no likelihood of exposure to sulfate and other aggressive chemicals. This is unfortunate because major markets for white and light colored concrete and concrete products exist in the coastal regions where exposure to high sulfate containing soils and seawater spray are likely.
Another additive that has been used in white cement is metakaolin. See for example U.S. Pat. Nos. 6,007,620, and 6,033,468, disclosing an interground white blended cement based on metakaolin. Metakaolin however, aside from its relatively high costs, differs from the pozzolan of this invention, in typically imparting a cream to pinkish tone to white cement and in having very high water demand, rendering it of limited commercial value in the area of present interest.
Now in accordance with the present invention the inventors have found that once it is ground to a powder of suitable fineness, the glass fiber waste discussed above can effectively function as a reactive pozzolanic admixture for use in portland cement-based building materials and products, such as concrete, mortars and grouts.
The inventors have also found that, unlike other cementitious and pozzolanic admixtures, such as blast-furnace slag, fly ash and silica fume which are characteristically dark in color, the finely ground glass pozzolan (which retains the vitreous nature and chemical composition of the fiber feed) is white in color. This white color makes the glass pozzolan highly desirable as a pozzolanic admixture for use with white portland cement, an application that is not suitable for the dark colored pozzolans.
According to a process aspect of this invention, glass fiber wastes are converted into high quality filler and pozzolan products, by shredding long entangled strands of glass into short fibers, adjusting the moisture content of the short fibers, grinding the short fiber, and classifying the ground material to produce a uniform high quality product with precise control over the maximum particle size and particle size distribution. Because of its physical characteristics, this product will at times herein be referred to as xe2x80x9cwhite VCAS pozzolanxe2x80x9d, the xe2x80x9cVCASxe2x80x9d being a reference to its production from fibers of xe2x80x9cvitreous calcium-alumino-silicatexe2x80x9d glass. The white VCAS pozzolan has a reflectance value of at least 80 as measured by a Technibrite TB-1C calorimeter according to the ISO 2467, 2471 method.
Several options are available for use of the white VCAS pozzolan of the invention in the cement and concrete industry: (a) it can be added as a separate pozzolanic component to the cementitious mixture (concrete, mortar, grout, etc.) meeting the requirements of ASTM C-618; (b) it can be blended in the powder form at the manufacturing plant with white portland cement to produce a blended cement meeting the requirements of ASTM C-595 or C-1157; and (c) it can be interground with white portland cement clinker and the appropriate amount of gypsum for set control at the finish mill. (The white clinker can e.g. be of Type I, Type II, Type III, Type IV, or Type V ASTM C-50 cements) In these functions, the new white VCAS pozzolans compare favorably with supplementary cementing materials such as fly ash, blast furnace slag, condensed silica fume, and metakaolin that are routinely sold into the cement and concrete additives markets. When blended with a portland cement, the white VCAS pozzolan will generally be present as 5 to 40% by weight of the total composition, with 10 to 30% weight being preferred. Those skilled in the art will also recognize that all of these applications for the white VCAS pozzolan are equally suited to grey portland cement, albeit without the advantage of light color. Accordingly there is no implied limitation of the use of the white pozzolan only in white cement systems. Where used in a blend with cement, conventional functional cement additives can also be present, such as accelerators, retarders, water-reducers/plasticizers, corrosion inhibitors, and pigments.
As a consequence of their fine particulate granulometry and a low surface absorptivity that provides low water demand, the VCAS pozzolans can markedly improve the compaction and cementing efficiency of cement and concrete mixtures in which it is used. As those skilled in the art will recognize, this has significant advantages for a designer of cement and concrete materials and products, because sufficient fluidity for placement can be achieved at a lower water content, and hence water-to-cement (w/c) ratio, thus allowing for either an increase in strength for a given cement factor, or a reduction in the cement content for a given strength. In addition, in common with other siliceous pozzolanic admixtures, the VCAS pozzolans react chemically in the cementitious matrix to consume excess calcium hydroxide, Ca(OH)2, produced from the hydration of the tricalcium and dicalcium silicates (C3S and C2S) in portland cement, converting it to calcium silicate hydrate (Cxe2x89xa6Sxe2x89xa6H) that functions as additional binding agent. The conversion of the calcium hydroxide to supplementary Cxe2x80x94Sxe2x80x94H binder serves to improve the long-term strength potential of cement and concrete products and materials, while at the same time improving their durability and service life towards deterioration in the presence of aggressive chemical media such as sulfate-containing soils and water.
As discussed above, the chemical composition of the raw materials used for the manufacture of white portland cement production is modified to achieve the light color, usually by reducing the iron and magnesium contents. As a result, white portland cements typically have higher tricalcium aluminate phase (C3A) contents than grey portland cements, as is shown in Table 2 based on information provided by the Portland Cement Association.
It is well known that it is the C3A content of a portland cement that is a measure of its susceptibility to attack by sulfate. Therefore, the higher content of this compound in white cement (C3A=11-12%) means that it will be more susceptible to this form of deleterious chemical attack than common Type I grey cement (C3A=8-10%). Historically, this chemical susceptibility has been one of the weaknesses of white portland cement that has limited its further growth into the market place, for example into areas where there is significant sulfate content in soils or in areas close to the ocean. While it is common practice to use pozzolanic admixtures such as fly ash, silica fume or blast-furnace slag to improve the durability of grey portland cement to deleterious chemical attack, these pozzolanic materials all impart a dark coloration to the cement matrix and are therefore unsuitable for use where a white color is required architecturally. Therefore, the availability of a white pozzolan is highly desirable for the production of more durable pozzolanic white cements.